1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device including a memory cell having a stacked gate structure.
2. Description of the Background Art
What has hitherto been known as a non-volatile semiconductor memory device uses a memory cell structure on which a floating gate is provided through a tunnel insulating layer on a semiconductor substrate, and a control gate is stacked thereon through an inter-layer insulating layer. The inter-layer insulating layer of this memory cell normally involves the use of a so-called ONO (Oxide-Nitride-Oxide) structure composed of a silicon oxide layer, a silicon nitride layer and a silicon oxide layer.
FIGS. 4A and 4B show sections, taken in two directions orthogonal to each other, of the memory cell structure described above. Normally in a flash memory, the control gates of a plurality of memory cells are consecutively arranged and serve as word lines. FIG. 4A is the section in the direction parallel to a direction of the word line.
An element isolation insulating layer 2 is provided on a p-type silicon substrate 1, and a floating gate 4 is provided through a tunnel insulating layer 3 on a device region defined by the element isolation insulating layer 2. Provided on the floating gate 4 is an ONO layer 5 as an inter-layer insulating layer including a silicon oxide layer 5a, a silicon nitride layer 5b and a silicon oxide layer 5c which are stacked in this sequence. Further, a control gate 6 is provided on the ONO layer 5. Source/drain diffused layers 7, 8 are provided in self-alignment with the control gate 6.
The ONO layer 5 functions to prevent electric charges accumulated in the floating gate 3 from leaking out during a writing process to the memory cell, and, because of a necessity for confining the electric charges within the floating gate 4 over a long period of time, is required to exhibit a high insulating property. In the normal flash memory, the floating gate retains electrons. In an electron accumulating state, however, a comparatively weak electric field (a self electric field) generated by the electrons is applied to the ONO layer 5.
The silicon oxide layer 5a, on the side of the floating gate 4, of the ONO layer 5, if a layer thickness thereof is 5–6 nm, works as a Fowler-Nordheim type tunnel current conductive mechanism, wherein the electric current flowing with a low electric field is extremely small. Further, a barrier height of the silicon oxide layer 5awith respect to silicon is as high as 3.2 eV. Accordingly, if the silicon oxide layer 5ahas no defect and there is no electric field enhancement effect based on a two-dimensional configuration of the floating gate 4, only the silicon oxide layer 5a must be capable of sufficiently retaining the electrons for a long time. In fact, however, there exist the defect and the two-dimensional electric field enhancement effect, and hence the ONO layer is used.
The two-dimensional electric field enhancement effect is typified by, for example, as indicated by a broken line A in FIG. 4A, an electric field enhancement at an edge which is obtained by forming the floating gate 4 in pattern. Further, there is an electric field enhancement caused by a rugged area formed on the surface of the floating gate 4 when the silicon oxide layer 5a is formed by thermal oxidation. The silicon nitride layer 5b of the ONO layer 5 contains much of trap level, and trapping occurs even when the electric current flows due to the electric field enhancement and acts to relieve the electric field, thereby restraining a leak of the electric charges from the oxide layer 5a surrounding the floating gate. If the oxide layer 5a has a defect, the same mechanism works. This is the reason why the silicon nitride layer 5b is used.
Incidentally, when the memory cell operates, and when in a state of the electrons being held by the floating gate, a positive bias is applied to the control gate 6. It is known that a large leak current flows to the silicon nitride layer through the trap level by hole conduction. Accordingly, supposing that the control gate 6 is provided directly on the silicon nitride layer 5b, the holes from the control gate 6 are injected, and therefore a dielectric strength is unable to be kept well. The silicon oxide layer 5c is provided upward in order to restrain the holes from being injected from the control gate 6.
The upper and lower silicon, oxide layers 5a, 5c of the ONO layer 5 are each required to have a thickness of 5–6 nm for exhibiting functions of relieving the electric field and preventing the leak. The silicon nitride layer 5b has a thickness on the order of 10 nm (converted into 5 nm in the case of the oxide layer). Hence, an equivalent oxide thickness of the ONO layer 5 is 15–16 nm thick.
There arise the following problems inherent in the inter-layer insulating layer based on the ONO structure described above.
First, it is desirable for enabling the memory cell to operate at a low voltage that a capacitance coupling between the control gate and the floating gate be large. It is desirable for attaining this that the ONO layer be as thin as possible. If the layer thickness is thinned down to a limit thereof, it can be thinned totally down to approximately 14 nm as an equivalent oxide thickness. However, further thinning of the layer becomes difficult.
Second, in the ONO layer, in a post-oxidating step after gate definition processing, as shown in FIG. 4B, a bird's beak B intrudes in a portion between the floating gate 4 and the control gate 6 from a side surface. The bird's beak decreases the capacitance coupling between the control gate 6 and the floating gate 4. Especially when the silicon oxide layer 5a disposed directly on the floating gate is provided by a CVD (Chemical Vapor Deposition) method, a characteristic of density thereof is inferior to a thermal oxide layer, and hence oxygen diffuses fast within the layer, with the result that a large bird's beak occurs. In the case of obtaining the silicon oxide layer at a low process temperature, the silicon oxide layer formed by the CVD method might be used more often than by the thermal oxidation, and therefore, in such a case, the intrusion of the bird's beak turns out a problem.